In recent years, a number of medical devices have been designed which are adapted for compression into a small size to facilitate introduction into the heart or a vascular passageway and which are subsequently expandable. These devices, among others, include septal occluders, stents and free standing filters which expand and are held in position by engagement with the wall of an organ or vessel. It has been found to be advantageous to form such devices of a shape memory material having a first, relatively pliable low temperature condition and a second, relatively rigid high-temperature condition. By forming such devices of temperature responsive material, the device in a flexible and reduced stress state may be compressed to fit within the bore of a delivery catheter when exposed to a temperature below a predetermined transition temperature, but at temperatures at or above the transition temperature, the device expands and becomes relatively rigid.
Originally, these implantable medical devices were intended to permanently remain in place, but recently it has become advantageous to retrieve the previously implanted device.
The development of removable implantable medical devices such as septal occluders, stents and filters which expand and are held in position by engagement with the wall of an organ or vessel has led to the development of intra vascular snares to retrieve these foreign bodies, usually from the peripheral vessels of the cardiovascular system. Single loop snares, such as those shown by U.S. Pat. Nos. 3,828,790 to Curtiss et al. and 5,171,233 to Amplatz et al. are commonly used snares. The Amplatz snare consists of a super-elastic nitinol cable with a single-formed loop. Because of the snare's super elastic construction, the loop can be introduced through small lumen catheters without risk of deformation. The loop is formed at approximately 90° to a cable, and this allows for the user to advance the loop over a foreign body and ensnare it by closing the loop with a small catheter. The foreign body is removed from the vasculature by withdrawing the device into a guiding catheter or vascular sheath.
In an attempt to provide a snare with improved cross sectional vessel coverage, multiloop snares such as those shown by U.S. Pat. Nos. 5,098,440 to Hillstead and 6,099,534 to Bates have been developed. These snares include loops which are joined only at their proximal ends to a shaft, and otherwise are not joined at any point between the shaft and the distal ends of the loops. This provides the advantage over single loop snares of enhanced cross sectional vessel coverage, and the free distal ends of the loops can be brought together to engage multiple surfaces of an intravascular medical device to be removed.
The problem with known snare recovery devices is that they are difficult to advance over a medical implant device and require skilled manipulation to retrieve an implanted device. Once the medical implant device is engaged by a recovery snare, there is no assurance that the device will not slip out of the snare during the recovery process.
It is particularly difficult to remove medical implants from the heart, such as septal occluders, with known snare recovery devices. Such snare recovery devices normally require appropriate sizing to the vasculature in order to facilitate successful ensnarement, and the geometry of multi loop snares is difficult to maintain during delivery. The relative position of the loops can change, both within a catheter or delivery tube and within a vessel, and the loops can actually become displaced or entangled during delivery.